In a standard system for monitoring the end of a helium purification or other process characterized by a sudden release of hydrogen into a helium gas stream, a small amount of air is added to the sample stream from a cryogenic adsorption column and the mixture is passed over a catalyst to form water as a function of hydrogen concentration. Heat generated during formation of water is proportional to the amount of hydrogen in the sample and increases the temperature of the gas exiting the cell. A thermopile containing approximately 20 thermocouples used as a temperature transducer has its "cold end" positioned in the gas stream entering the catalytic cell and its "hot end" positioned in the gas exiting the cell. In the absence of hydrogen, variations in the flow rate of the gas stream as well as variations in ambient temperature cause the output of the thermopile to drift due to its electrothermal asymmetry. The output of the thermopile also contains hash (high frequency noise) caused, among other factors, by current transients developed during switching of electric motors and other heavy electrical equipment. The end of the process is identified by a relatively abrupt output signal generated by the thermopile in response to heat generated following hydrogen release. The hydrogen responsive signal generated by the thermopile is termed herein the transducer end point or data component.
Whereas the period of the drift component is much greater than the period of the end point component of the thermopile output signal, the amplitudes of the two components may be comparable to each other. The hash component typically is at a much lower period with an amplitude sometimes far exceeding the amplitude of the end point component. In practice, a level detector is generally provided to respond to the transducer data or end point signal in the presence of drift and hash to energize an alarm and thereby indicate to a human operator that the end of the process has occurred. In response, the operator manually switches the helium gas stream to a fresh adsorption column and the hydrogen-free sample stream passes through the cell to allow the system to recover. Since the amplitude of the drift signal component may be equal to the amplitude of the data signal component and the amplitude of the hash component may be much greater, however, the data may be swamped so that it is impossible to detect the end point swing of the thermopile.
Whereas the hash component of the composite transducer signal is often eliminated by low pass filtering, the signal processing system must be periodically rebalanced or the drift component alone will exceed the threshold setting of the alarm detector and a false alarm will occur. Intermittent automatic zeroing such as is used in conventional chromatographic systems requires a finite time period for zeroing and thus prevents the system from providing continuous monitoring of the industrial process.
Accordingly, it is an object of the present invention to provide a method of and system for reducing drift and hash in a transducer output signal.
Another object is to provide a method of and system for processing a parameter responsive transducer signal in an end point detection system of a type incorporating a transducer having an output that tends to drift over a period of time.
Another object is to provide a method of and system for drift compensation in an end point detection system of a type using a transducer having an output that drifts very slowly over a period of time and has a drift amplitude that is comparable to the amplitude of a data signal component generated by the transducer.
An additional object of the invention is to provide a method of and system for continuously and automatically zeroing the output of a transducer having a tendency to drift as a function of ambient conditions.